Blood contact with biomaterials during cardiopulmonary bypass(CPB) activates at least five plasma protein systems and five blood cells that produce the vasoactive substances and microemboli that mediate the bleeding, thrombotic and inflammatory complications associated with open heart surgery. The rationale of this proposal is to prevent these blood reactions by inhibiting the function of key blood elements using reversible inhibitors during the period of CPB. We use three models: an in vitro system (SECC), baboons, whose blood proteins cross-react with antibodies against human antigens, and patients. Because thrombin forms and circulates during CPB in every patient despite heparin, one goal of this proposal is to prevent formation and circulation of thrombin with recombinant tick anticoagulant peptide or enoxaprin directed against factor Xa alone or in combination with direct inhibitors of thrombin, r- hirudin or DuP 714 (Bz-Phe-Phe BoroArg chloromethyl ketone) in our in vitro and baboon models. A second goal is to determine the role of tissue factor expressed in the wound and/or in monocytes during CPB in stimulating thrombin formation via the extrinsic coagulation pathway. A third goal is to determine the relative importance of the extrinsic and intrinsic coagulation pathways by studies of tissue factor expression and factor VIIa generation and by studies of contact system activation using new, specific intermediates: kallikrein--2-macroglobulin complex, kinin- free kininogen and indicators of high and low molecular weight kininogen cleavage in patients. To reduce factor Xa formation, we will use recombinant tissue factor pathway inhibitor (r-TFPI) or tissue factor antibody to block the extrinsic pathway and a new potent peptide, ecotin, to block the intrinsic pathway. A fourth goal is to resolve the controversy as to whether or not circulating platelets are functionally competent by using sensitive antibodies against different conformations of the platelet GPIIb/IIIa receptor. Additionally, we will test two synergistic, reversible platelet inhibitors in combination in the baboon to achieve "platelet anesthesia" during CPB. This proposal utilizes our ability to measure a wide variety of blood constituents and reaction markers to understand the mechanisms of blood activation during CPB. With this knowledge, we can use our in vitro and baboon models to develop inhibitors of selected, specific reactions that mediate the bleeding, thrombotic and inflammatory complications associated with CPB.